The invention relates to pre-press, and in particular to preparing data for repetitive imaging of a screened image on a flexographic printing sleeve or roller.
At present, flexography is one of the main printing processes. A flexographic (“flexo”) sleeve, normally rubber or photopolymer, is fabricated in such a way that the areas corresponding to zones to be inked are geometrically higher than the areas corresponding to zones not to be inked. Contacting the flexo sleeve with an inking roller, such as an anilox roller, inks the flexo sleeve. Only the geometrically higher zones of the flexo sleeve are inked, other areas are not inked. Subsequently, the inked flexo sleeve is brought in contact with a substrate and the inked parts transfer ink onto the substrate, thus producing the desired image on the substrate.
In flexography, there is demand for printing continuous designs such as wallpaper, decoration and gift wrapping paper. In general, such flexography applications use a cylindrical form, usually a printing sleeve or a cylindrical printing cylinder formed by fusing the edges of a sheet together to form a seamless, continuous element. Such continuous printing elements are well suited for mounting on conventional laser exposing equipment (“imagers”) such as the Esko-Graphics Cyrel® Digital Imager (Esko-Graphics, Gent, Belgium) or flexography engravers available from ZED Instruments Ltd. (Hersham, Surrey, England) or Applied Laser Engineering Ltd. (West Molesey, Surrey, England).
Many designs typically need to be screened (halftoned) before being output on an imagesetter or engraver. Screening of separated data is performed using a Raster Image Processor (RIP), that converts the continuous-tone (contone) data of each of a set of color separations into a binary image, e.g., a bitmap by replacing the “gray” values in the original color separation data by screening dots of proper area density. FIG. 1 shows a color separation 100 after screening with an area 102 enlarged so that the screening pattern of screening dots such as dot 104 is visible. Most modern screens are defined by a screen resolution, a screen angle, a line-count (also called screen ruling), and a dot shape. Each of the color separations is thus screened. The characteristics of the different colors' screens are chosen so that they can combine to form a color image having negligible artifacts such as Moire patterns.
For continuous designs, it is desirable to select the screen characteristics so that the screen repeats exactly with the circumference of the sleeve or roller in the fast-scan direction of the imager, e.g., the circumferential direction in the case of a drum imager. The circumferential zero-line position is the line of repetition, i.e., the circumferential position where the imaging commences on the sleeve of the drum, and every circumferential position that is an integral number of rotations away. If the screen does not repeat with the circumference, the mismatch between the screening on the sides of the line of repetition—the circumferential zero-line—results in a visible seam in the print. The circumferential zero-line is thus also called the seam herein. FIG. 2A shows a drum 202 with a sleeve or roller 204 attached. FIG. 2B shows in enlarged form the image 208 on the drum region that includes the circumferential zero line shown as 206. The circumferential direction is shown as the fast-scan direction 212 and the axial direction as is the slow-scan direction 214. As can be seen, unless care is taken, an undesirable mismatch pattern forms at the seam.
Special screens and screening techniques have been developed for continuous printing to prevent the appearance of such a visible seam. However, using such special “seamless”screens may have some limitations. For example, forcing the seamless property may restrict the combinations of line-counts, imaging resolution, and dot shapes that are available. Furthermore, there are many RIPS already installed that are not able to generate such seamless screens. Furthermore, there the separations may include one or more areas of pre-scanned, pre-screened art, and for such art, the screens are pre-determined.
There thus is a need for a method to process a set of already screened separations, at least one of which has a mismatch in the screening pattern along the circumferential zero-axis of the sleeve or roller in order to either eliminate or at least hide the mismatch so that continuous designs may be printed without a visible seam.